Catalytic gas heater screen system

ABSTRACT

A system for oxidation of carbon monoxide produced from gas log sets used in a vent free mode in a wood burning fireplace or fireplace insert. In the system a vent chamber is placed in the top of the combustion chamber. The vent chamber has three sides, a ceiling and a floor the front of the chamber being open and venting outside of the firebox. The vent chamber is positioned in the top of the fireplace combustion chamber. The floor of the vent chamber has a hole cut into it and a low shape profile, low pressure drop catalytic screen placed in the hole so that the exhaust can pass through the catalyst into the chamber and then vent to the room. A curtain may be placed below the chamber vent and in front of the catalyst to act as a baffle or dam to restrict the flow of rising hot exhaust and redirecting it through the catalyst.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] 2 . Related Art

[0003] Vent free gas burning appliances are growing in popularity and use. The high thermal efficiency, low cost, ease of installation and the cleanliness of these heating devices make them attractive to consumers. The downside of the use of vent free products is that the products of combustion such as carbon dioxide, water and sulfur oxides can be detriments to indoor air quality and oxygen depletion and carbon monoxide emissions are dangerous. Several manufacturers supply vent free gas fireplace products with oxygen depletion sensors and oxidation catalysts for carbon monoxide destruction. In order to make the catalyst functional the air must be directed through the catalyst structure. This is accomplished by enclosing the combustion chamber with a glass or glass ceramic front surface and placing the catalyst so that the normal drafting process produces flow through the catalyst. This arrangement produces exhaust with essentially no carbon monoxide.

[0004] Vent-free gas fireplaces, stoves and vent-free log sets have been available for a number of years. In the past few years numerous catalytic vent-free versions have been introduced. The advantages of catalytic vent-free versions lie in the improved safety afforded by the presence of the catalyst to reduce carbon monoxide emissions. The catalyst is usually supported on large cell honeycomb structure that may be either ceramic or metal in composition. Because this structure creates a flow resistance, however small, the catalytic vent-free appliance requires a closed combustion chamber in order to force the flue gases through the catalysts. In order to maintain the aesthetics of the appliance, a high temperature glass door is utilized.

[0005] Many more gas fireplaces are in service, which use the so-called gas log set. These sets are made from a burner arrangement, which produces attractive flickering flames over artificial ceramic logs. Many of these log sets have been designed to be placed in a wood burning fireplace and to have the exhaust removed by a chimney. Still others have been designed to be placed in a fireplace insert or a wood-burning fireplace in which the damper is sealed to produce a vent free arrangement. These arrangements give some of the advantages of the vent free products are not known for use with a catalyst to destroy carbon monoxide due to the open nature of the combustion chamber.

[0006] The present invention employs a very low pressure drop catalyst coupled with a fully or partially catalyzed screen to eliminate the need for the expensive high temperature glass and yield a more conventional fireplace look.

SUMMARY OF THE INVENTION

[0007] The present invention is a system to provide for oxidation of carbon monoxide produced from gas log sets used in a vent free mode in a wood burning fireplace or fireplace insert. The system comprises a vent chamber positioned at the top of a combustion chamber. The vent chamber has three sides, a top and a floor. The front of the vent chamber is open and vents outside of the combustion chamber. The floor of the vent chamber which is contiguous with the combustion contains an opening. A low shape profile, low pressure drop catalytic structure is placed in the opening in a frame so that a portion of the exhaust passes through the catalytic structure into the vent chamber and then vents to the room. A curtain may be placed below the vent of the vent chamber and in front of the catalytic structure to act as a baffle or dam to restrict the flow of rising hot exhaust and redirecting it through the catalytic structure.

[0008] In a preferred embodiment the invention is a system to provide for oxidation of carbon monoxide produced from gas log sets used in a vent free mode in a wood burning fireplace or fireplace insert comprising a vent chamber positioned at the top of a combustion chamber, said vent chamber having three sides, a top, a floor having an opening therein for gaseous communication with said combustion chamber, an open front to vent outside of the combustion chamber having a mouth, a catalytic structure is positioned in the opening in a frame so that a portion of exhaust from burning within the combustion chamber passes through the catalytic structure into the vent chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a schematic front elevational view representation of a fire box employing one embodiment of the present invention.

[0010]FIG. 2 is a schematic side view elevational view of an embodiment having a heat defector.

[0011]FIG. 3 is a top view of a filter in a frame.

DETAILED DESCRIPTION

[0012] The vent chamber height ranges from 0.5 to 6 inches. In one embodiment the vent chamber is formed as a portion of a fireplace insert.

[0013] The curtain may be made of stainless steel that has been oxidized, coated with a catalytic coating and mounted hanging from the front of the chamber floor into the combustion chamber. The curtain size may be height of the fireplace open face. The curtain may be a knitted or woven metal screen typically used as a fireplace curtain and may hang on a sliding rod and can be pulled aside to expose the flame. The curtain may also be coated with a catalytic coating. The curtain may also be made of non porous metal sheet or foil.

[0014] Preferably the catalytic structure comprises a screen. The screen is catalyzed either over a portion or over its full length in order to reduce the level of carbon monoxide in the gas that escapes through the screen. The screen may consist of any type of structure, be it woven, knitted, or otherwise, which preferably has more than 30% open area, to which the catalytic material can be fixed. More preferably the screens have more than 6 meshes per square inch and less than 600 meshes per square inch and open area of 35 to 75%. The screens, where more than one screen is employed, may be the same or of different mesh and open area.

[0015] The catalytic structure is preferably one or more screens made of stainless steel which have been oxidized, coated with a catalytic coating and mounted in the hole in the chamber floor. The catalyst coating is preferably a high surface area ceramic chosen from aluminum oxide, cerium oxide, titanium oxide, zirconium oxide or mixtures thereof and coated with catalytic components comprising a noble metal, preferably platinum, palladium, cobalt or mixtures thereof.

[0016] Referring to FIG. 1 a schematic representation of a conventional firebox 10 having an interior vent 14 is depicted. A gas log set 12 is frequently used to provide heat. This conventional arrangement, however must rely on precise and careful adjustment of the flame to keep carbon monoxide levels low. In FIG. 2 this conventional setup has been modified in accordance with the present invention with a filter 18 seated on an open or perorated structure 28 which allows passage of the combustion gases through the filter and into the vent chamber 24. An extension 16 may be added to the original vent if necessary to direct the filtered gas to the vent chamber for distribution. This feature is particularly related to modified conventional fireboxes. Preferably a curtain or dam 26 extends from the filter bearing structure to aid in channeling the combustion gases thru the filter rather than out of the open face 30 of the firebox.

[0017] In FIG. 3 a preferred form of filter comprising a frame 20 which is adapted to fit over or into opening or perorations in the structure 28 which contains a low pressure drop catalyst such as a metal wire mesh coated with and oxidation catalyst.

EXAMPLE 1 Control

[0018] A gas log set rated at 32,000 BTU/hr was connected to propane fuel tank and placed into a fireplace insert as shown in FIG. 1. The fire was lit and the emissions were monitored for oxygen, carbon dioxide and carbon monoxide. Oxygen and carbon dioxide levels were determined by an Illinois instrument model # 3530 and carbon monoxide was determined using a TECO model # 48 non dispersive infrared analyzer. The exhaust was sampled at the outlet of the overhanging hood just as the gases exhaust from the top of the firebox as shown in FIG. lb. The sample probe was constructed of stainless steel .75 inch diameter pipe which, was 36 inches long and held eight one eighth inches diameter tubes each 3 inches long and spaced 4 inches apart. The sample was extracted by a diaphragm pump and forced into a gas dryer and then into the analytical instruments for gas analysis. The carbon monoxide values were corrected to moisture and oxygen free values by the following calculation:

Air Free CO=CO measured×13.76/Carbon Dioxide measured

[0019] In the first experiment the measured CO is as shown in Table 1. TABLE 1 Baseline Carbon Monoxide Levels. Oxygen and Moisture free Time, minutes CO ppm CO₂ % Calculated CO ppm 0 0.5 0.1 1 16.6 0.8 276.9 2 15.8 1.0 210.5 3 15.8 1.0 210.5

[0020] On a moisture free and oxygen free basis the Carbon Monoxide levels were 210.5 ppm.

EXAMPLE 2

[0021] An experiment was carried out as in Example 1 except that a chamber was produced by inserting a frame which was constructed to hold one layer of stainless steel wire mesh with 30 meshes per inch. The screen had been coated with 1 % Pt on alumina wash coat which had been milled to less than 2 micron diameter and applied by spray coating. The catalyst coating was treated at 500° C. for one hour and the coated screen was fitted to the frame. The frame was placed in the top of the fireplace insert to allow a 1.5 inch space between screen and ceiling. The carbon monoxide reading was 8.6 ppm and carbon dioxide was 1.5%. When these data were corrected to oxygen and water free conditions the carbon monoxide was 78.8 ppm indicating a 62% destruction of carbon monoxide.

EXAMPLE 3

[0022] The same experiment was performed as in Example 2, except that 2 layers of 30 mesh, catalyzed screen was installed. The carbon monoxide was measured at 2.4 ppm 1.6% CO₂. The calculated oxygen free and water free carbon monoxide level is calculated at 20.6 ppm indicating a 90% destruction of carbon monoxide.

EXAMPLE 4

[0023] The same experiment as in Example 3 was performed except that a stainless steel foil 1.5 inches wide was placed below and in front of the catalyzed screens to force more of the exhaust to go through the screen as shown in FIG. 2. The measured carbon monoxide was 1.3 ppm and the CO₂ was 1.7%. Calculated oxygen and moisture free carbon monoxide was 10.5 ppm. This indicates a carbon monoxide destruction efficiency of 95%.

EXAMPLE 5

[0024] The same experiment was performed as in Example 2, except that 2 layers of 18 mesh, catalyzed screen was installed. The carbon monoxide was measured at 3.9 ppm and carbon dioxide was 1.5%. The calculated oxygen free and water free carbon monoxide level was 35.8 ppm indicating an 82.9% destruction of carbon monoxide.

EXAMPLE 6

[0025] The same experiment was performed as in Example 5, except that 3 layers of 18 mesh, catalyzed screen was installed. The carbon monoxide was measured at 2.5 ppm and carbon dioxide was 1.4%. The calculated oxygen free and water free carbon monoxide level was 24.6 ppm indicating an 88.3% destruction of carbon monoxide.

EXAMPLE 7

[0026] The same experiment was performed as in Example 5, except that the space between the catalyzed screens and the ceiling was increased to 2 inches. The carbon monoxide was measured at 4.5 ppm and carbon dioxide was 1.6%. The calculated oxygen free and water free carbon monoxide level was 38.7 ppm indicating an 81.5% destruction of carbon monoxide.

EXAMPLE 8

[0027] The same experiment was performed as in Example 6, except that the space between the catalyzed screens and the ceiling was increased to 2 inches. The carbon monoxide was measured at 2.25 ppm and carbon dioxide was 1.5%. The calculated oxygen free and water free carbon monoxide level was 20.6 ppm indicating a 90.1% destruction of carbon monoxide.

EXAMPLE 9

[0028] The same experiment was performed as in Example 8, except that a 1.5 inch wide stainless steel foil was placed in front of and beneath the catalyzed screens to force more of the exhaust through the catalyzed screens. The carbon monoxide was measured at 1.6 ppm and carbon dioxide was 1.5%. The calculated oxygen free and water free carbon monoxide level was 14.6 ppm indicating a 93% destruction of carbon monoxide.

EXAMPLE 10

[0029] The same experiment was performed as in Example 2, except that 2 layers of 16 mesh, catalyzed screen was installed. The carbon monoxide was measured at 6.3 ppm and carbon dioxide was 1.4%. The calculated oxygen free and water free carbon monoxide level was 47.2 ppm indicating a 78% destruction of carbon monoxide.

EXAMPLE 11

[0030] The same experiment was performed as in Example 10, except that 3 layers of 16 mesh, catalyzed screen was installed. The carbon monoxide was measured at 4.5 ppm and carbon dioxide was 1.4%. The calculated oxygen free and water free carbon monoxide level was 19.2 ppm indicating a 79% destruction of carbon monoxide.

EXAMPLE 12

[0031] The same experiment was performed as in Example 10, except that a 1″ stainless steel curtain was installed to hang down at the forward edge of the catalyzed screens. The carbon monoxide was measured at 5.8 ppm and carbon dioxide was 1.5%. The calculated oxygen free and water free carbon monoxide level was 41.3 ppm indicating an 80% destruction of carbon monoxide.

EXAMPLE 13

[0032] The same experiment was performed as in Example 10, except that a 1.5″ stainless steel curtain was installed to hang down at the forward edge of the catalyzed screens. The carbon monoxide was measured at 5 ppm and carbon dioxide was 1.5%. The calculated oxygen free and water free carbon monoxide level was 33.9 ppm indicating an 84% destruction of carbon monoxide. This shows the improvement with use of a curtain.

[0033] The results of these examples are summarized in Table 2 below TABLE 2 Summary of Data from Examples # of Screens EX. Mesh Size Air Dam CO Destruction, % 1 CONTROL 0 2 30 1 No 62 3 30 2 No 90 4 30 2 Yes 95 5 18 2 No 82.9 6 18 3 No 88.3 7 18 2 No 81.5 (2″ vs 1.5″ space) 8 18 3 No 90.1 (2″ vs 1.5″ space) 9 18 3 Yes (1.5″) 93 (2″ vs 1.5″ space) 10  16 2 No 78 11  16 3 No 79 12  16 3 Yes (1) 80 13  16 3 Yes (1.5″) 84 

The invention claimed is:
 1. A system to provide for oxidation of carbon monoxide produced from gas log sets used in a vent free mode in a wood burning fireplace or fireplace insert comprising a vent chamber positioned at the top of a combustion chamber, said vent chamber having three sides, a top, a floor having an opening therein for gaseous communication with said combustion chamber, an open front to vent outside of the combustion chamber having a mouth, a catalytic structure is positioned in the opening in a frame so that a portion of exhaust from burning within the combustion chamber passes through the catalytic structure into the vent chamber.
 2. The system according claim 1 wherein the catalytic structure comprises at least one screen.
 3. The system according to claim 2 wherein the screen has more than 30% open area.
 4. The system according to claim 3 wherein the screen has more than 6 meshes per square inch to less than 600 meshes per square inch and open area in the range of 35 to 75%.
 5. The system according to claim 2 wherein catalytic structure comprises more than one screen.
 6. The system according to claim 2 wherein said screen is coated with a high surface area ceramic chosen from aluminum oxide, cerium oxide, titanium oxide, zirconium oxide or mixtures thereof and a catalytic component of platinum, palladium, cobalt or mixtures thereof.
 7. The system according to claim 2 wherein said screen is coated with a catalytic coating is composed of a high surface area oxide coating and a noble metal catalytic component.
 8. The system according to claim 7 wherein the noble metal comprises Pt. 9 The system according to claim 1 wherein said curtain extends below said frame across said mouth.
 10. The system according to claim 9 wherein said curtain is porous.
 11. The system according to claim 9 wherein said curtain is non porous.
 12. The system according to claim 10 or 11 wherein said curtain is metallic. 